The present invention relates to novel polymeric materials from insoluble polymers originally containing the pendant nitrile group, the conversion of a substantial fraction of the surface groups to amides, the further conversion of surface amides to substituted amides, and the uses of these materials, and also to insoluble polymers containing pendant ester groups which by other reaction sequences produce pendant amide groups on their surfaces.
The known methods for the treatment of polymeric surfaces, fibers, membranes and the like to impart to them hydrophilicity or a high degree of water-wetability have involved reactions much as the hydrolysis of esters to alcohols and acids by bond cleavage, and also the sulfonation of olefinic, aromatic and other polymers to place sulfonic acid groups on the surface. The surface groups responsible for hydrophilicity have been ionic or polar in character. The grafting of polar monomers onto the surface of water-insoluble polymers to render the surface hydrophilic is also well known. These surface treatments can render the surface polymer chains partly water-soluble, or the polymer itself is converted to a gel-like form and partially solubilized by such treatment, or long hydrophilic chains extend into the water and make for undesirable properties in the formed product.
Particularly desirable, very strongly hydrophilic polymers are those which have an extensive fraction of their surface groups as pendant amides not joined to an aromatic ring. The strongly hydrophilic and useful polymer polyacrylamide and its ionic derivatives are examples; a recent patent by Gregor et al. (U.S. Pat. No. 45,596,858, Jun. 24, 1984) and a co-pending CIP describe the insolubilization of polymers containing the pendant aminocarbonyl and other groupings. These water-soluble polymers were then insolubilized by a novel cross-linking reaction so that they could be made in the forms of films, filters, membranes and the like.
In the present invention water-insoluble polymers are treated on their superficial and pore surfaces to place substantial amounts of pendant amides thereupon. Water-insoluble film-forming polymers containing the nitrile group such as polyacrylonitrile (PAN) are treated to convert a substantial fraction of their surface nitrile groups to the amide in a reaction which does not alter the morphology and other properties of the polymer appreciably but does render it strongly hydrophilic. The reactivity of the amide moiety is such that derivatives can be made to impart to the surface other, desirable properties. The present invention provides for a surface treatment which is unique in the extent and hydrophilicity of the final product, its practicality of application and the wide range of applications which result. These surface reactions are believed to proceed by the partial hydrolysis of the nitrile groups by the use of either alkaline hydrogen peroxide or the use of a strong acid as by mixtures of sulfuric acid with other materials to produce substantial amounts of the amide group. The reaction which employs hydrogen peroxide makes use of a reducing agent which eliminates the production of oxygen and subsequent blistering to make for a strongly hydrophilic surface which retains the essential physical properties and structure of the original material.
The novel surface treatment techniques taught by this invention produce microfiltration and ultrafiltrations membranes and the like with surfaces which do not become readily fouled or denature proteins. Typical microfilters and ultrafilters are cast from or formed from water-insoluble polymers which are usually rigid and physically strong but are hydrophobic and are readily fouled by the adsorption of hydrophobic particles and solutes from feed streams. Such fouling is a major problem in the industrial use of membranes and filters leading to low rates of throughput and making frequent cleansing or costly pretreatment necessary. Materials produced by the selective surface treatment of nitrile-containing and other polymers as taught by this invention have been shown to resist such adsorptive fouling while retaining their insolubility in water and their useful physical characteristics, including their porosity.
The teachings of this invention are applicable to the preparation of many different kinds of materials. For example, it has been found that surfaces which are relatively smooth (on a molecular level) and formed by the dissolusion and complete drying of a water-insoluble polymer such as PAN or from PAN fibers, are converted relatively slowly into hydrophilic materials by the surface treatments described herein. On the other hand, the same surface treatments applied to polymers which have been coagulated from solution, where the surface area is quite large (per macroscopic unit area of membrane) and where the porous surface is quite rough are made hydrophilic rapidly. The materials which can be rendered useful by the surface treatments include screens, microfilters and ultrafilters, all of which retain the high physical strength of the base polymer.